1. Field of the Invention
The present invention relates to the forming of an HF (high frequency)-control switch.
2. Discussion of the Related Art
U.S. Pat. No. 6,862,196, which is incorporated herein by reference, relates to a thyristor control structure comprising an HF (high frequency) control source connected via a transformer to the gate-cathode circuit of a thyristor, a diode being interposed in this gate-cathode circuit.
It is disclosed in the above-mentioned US patent that, if the HF frequency is high enough, a miniaturized transformer formed on a glass substrate may be used. In this patent, examples of operating frequencies on the order of 40 MHz or more are given. Various advantages of this control mode are mentioned: transformer miniaturization, simplified assembly of the transformer and of the thyristor, improvement of the insulation, control of head-to-tail thyristors etc.
It is also known, as illustrated in FIG. 1, to provide an assembly of the same type to control a bi-directional switch 1 having its gate-cathode circuit connected to a secondary winding of a transformer T with an interposed series diode D.
As illustrated in FIG. 2, bi-directional switch 1 of FIG. 1 may be a bi-directional switch of the type having its control referenced to the rear surface terminal. Such a component is schematically and partially illustrated in the cross-section view of FIG. 2. It comprises a power portion comprising NPNP and PNPN thyristors in parallel. These two thyristors are formed in a structure comprising a lightly-doped N-type substrate 10. On the lower and upper surface sides are formed P-type layers, 11 and 12, layer 11 being on the upper surface side and layer 12 on the lower surface side. In region 11, an N-type region 13 on the left-hand side of the drawing is formed so that a vertical NPNP thyristor is formed of region 13, of layer 11, or substrate 10, and of layer 12. An N-type 14 layer is formed on the rear surface side at least in front of the portion of P-type region 11 in which N region 13 is not formed. A vertical PNPN thyristor comprising regions and layers 11, 10, 12, and 14 is thus formed. In this type of component referenced to the rear surface, the gate is formed of an N+-type region 16 formed on the upper surface side of a peripheral well or diffusion wall 18 surrounding the entire bi-directional switch.
It should also be noted that U.S. published patent application No. 2005/0082565 of the applicant describes an HF control of a bi-directional component structure. This patent application specifically describes the case where the bi-directional component structure comprises a gate terminal and a gate reference terminal. The control signal is then applied between these terminals, the gate reference terminal being distinct from the main front surface terminal.
It is often attempted to decrease the sensitivity of bi-directional components to spurious triggerings linked to voltage peaks (dV/dt sensitivity) and to improve their switching performances, that is, in particular, for the conduction during a halfwave not to cause a conduction during a next halfwave of opposite polarity (di/dt sensitivity to switching). This, however, poses a problem in that, when a bi-directional component is desensitized to make it less sensitive to noise, it is also less sensitive to a normal control signal. This problem is all the more acute when the frequency of the HF control signal is desired to be increased, the power contained in each halfwave then being lower.